"To deal with a group like that, you need a carrot and stick. The carrot is finding out how to reach out to them," he said. "When you try to reach out to them and they are not amenable to being reached out to, you have to use the stick."

Obasanjo said President Goodluck Jonathan was "just using the stick" in his efforts. "He's doing one aspect of it well, but the other aspect must not be forgotten."

"If they had 25% support a year and a half ago, today that support has doubled," the former president said.

Analysts suggest that reaching out to Boko Haram may be increasingly difficult because the group has split into different factions, some with a domestic focus and others with a more pan-jihadi approach.

Resolving the issue is key to Nigeria's progress, according to Obasanjo, who now heads an eponymous foundation that is working to promote human security across Africa.

In a report published late last year, Amnesty International condemned the increasingly brutal attacks carried out by Boko Haram since 2009, but said Nigeria's security forces "have perpetrated serious human rights violations" in response. A military spokesman rejected the allegations.

The militant group, whose name means "Western education is forbidden," is fighting to impose a strict version of Sharia law in the northern part of the country.

In the past, the group attacked other Muslims it felt were on an immoral path, but it has increasingly killed Christians.

The U.S. State Department has accused Boko Haram of attacking mosques and churches to incite tensions between the two religious groups, hoping to drive a wedge between them. It has condemned some of the group's leaders for alleged ties to al Qaeda.

Nigeria has almost equal numbers of Christian and Muslims, with the south predominantly Christian. Boko Haram and other Muslim groups say the north has been starved of resources and marginalized by the government of Jonathan, a Christian.